Duchenne and Becker muscular dystrophies (DMD/BMD) are X-linked recessive genetic disorders caused by mutations in the dystrophin gene. DMD is among the most common lethal inherited disorder of children, but finding a cure remains an elusive goal. Improved treatment approaches using surgery, corticosteroids, respiratory support and heart medication are leading to increases in lifespan. While most patients previously died of respiratory failure due to weakness of the diaphragm and intercostal muscles, a shift has been observed to an increasingly older patient population dying of respiratory or heart failure. Nonetheless, most treatment strategies for DMD/BMD are focused on limb muscles, despite evidence that skeletal muscle rescue can exacerbate cardiac dysfunction. Our lab has shown that adeno-associated viral (AAV) vectors can efficiently target both skeletal and cardiac muscles in mouse (mdx) and canine models for DMD, and that systemic AAV delivery of micro-dystrophin cassettes can halt progressive dystrophic changes and significantly improve diaphragm and cardiac function in mdx mice. Functional rescue, while impressive, is incomplete and recent studies suggest multiple ways to improve the functionality and muscle-specific expression of micro-dystrophin. Testing and comparing these modified micro-dystrophins is complicated by the mildness of the mdx mouse phenotype, so we propose to use a more severely affected mdx4cv/Dba-2 mutant that displays extensive cardiorespiratory fibrosis and a more progressive pathology. These cassettes will comprise both new microdystrophin cDNAs and new gene regulatory cassettes optimized for expression and function in respiratory and cardiac muscle following systemic AAV-mediated gene transfer to adult animals. We will also integrate these delivery systems in the context of strategies designed to prevent immune-mediated loss of dystrophin expression. These include a novel reverse vaccine protocol designed to induce tolerance, and transient immune suppression. Together these approaches could significantly impact the ability to treat respiratory and cardiac muscles of DMD patients. This work also has broad implications for gene therapy of other disorders involving cardiopulmonary dysfunction.